It is well known that internal combustion engines generate noise over a range of sound frequencies during engine operation. Such noise can originate from mechanical, hydraulic, and/or pneumatic actions of various engine and auxiliary components, and amplitude of the noise can be a function of the revolutionary speed of the engine. In engine applications such as powering a vehicle, other components such as transmissions, brakes, and the like can also contribute to the overall noise level. It is generally recognized as being desirable to minimize the audible noise emitted by an engine and/or corresponding vehicle under all conditions of operation. Noise frequencies of interest are typically within the range of less than 660 Hz.
It is further known that some of these various engine and vehicle components may have one or more natural harmonic frequencies, and that those components can be excited to resonate undesirably when their resonant frequencies are also present in an engine and/or vehicle sound emission spectrum, thereby amplifying those frequencies and increasing the overall level of perceived noise.
A particular example of a resonant component is a rotatable shaft of an airflow tuning valve in a tunable intake manifold. In an exemplary prior art embodiment, the valve shaft has a free resonant length of about 226 mm between restraining bearings and a resulting natural resonant frequency of about 300 Hz. Air flowing past the central portions of the shaft can cause the prior art shaft to resonantly respond like a reed undesirably at this frequency.
What is needed in the art is dampening mechanism for reducing the natural resonant response of a shaft used in a vehicle powered by an internal combustion engine.